Bug 32044: Improve consistency in primary submit buttons

Bug 32044: Improve consistency in primary submit buttons

We currently have a mix of ways to style buttons, including some using
Bootstrap's btn-primary class and some with CSS targeting
input[type='submit']. This patch works to make these buttons more
consistent in their appearance and to improve the modularity of related
SCSS.

The patch also removes some unused CSS from login.css.

To test, apply the patch and rebuild the staff interface CSS.

Test various page to see if main submit buttons look correct in their
default, hover, and active states:

- The login page
- Circulation -> Checkouts -> Check out button (btn-primary)
- Circulation -> Set library (input[type='submit'])
- Cataloging -> Basic MARC editor (split button)

Also check "default" buttons:

- Patrons -> Sidebar filter (input[type='button'])
- Circulation -> Checkouts -> Show checkouts (btn-default)

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>